A rudimentary familiar example of a counterbalance is represented by a simple coil spring (or a pair of coil springs at spaced-apart hinges) used for maintaining the hood of an automobile in its raised position. Such a spring counterbalance may be used to illustrate the fundamental properties of such devices. When a hood equipped with a spring counterbalance is to be lifted from its horizontal closed position, commonly much of its weight is not counterbalanced so that considerable effort is required. With the hood in its raised position, it is common for counterbalance springs to overcompensate for the weight of the hood.
Compression coil springs have basic characteristics of exerting increasing force as they become progressively more compressed, and of having a more-or-less constant rate, i.e., requiring a constant increase in force for each increment of compression. These characteristics are taken into account poorly in common spring counterbalances.
Compression coil-spring counterbalances ordinarily occupy a large volume where a large weight is involved. In some applications of counterbalances where space is severely limited, efficient utilization of space is important.
Compression coil springs are considered quite dependable, and yet they do fracture occasionally. Such catastrophic failure of a spring counterbalance can have serious consequences. A heavy lid can slam suddenly, causing grave personal injury.
The same attributes of coil springs are limitations in other spring devices, such as compression coil springs used as "suspensions" in automobiles.
The foregoing limitations are ideally taken into account in an illustrative form of counterbalance shown and described in my prior application Ser. No. 422,838, filed Sept. 24, 1982 and Ser. No. 477,337 filed Mar. 21, 1983, both now abandoned. In that illustrative embodiment, a bundle of resilient rods is subjected to torsion. End portions of the rods are keyed in sockets that are arrested against turning. One or more cam-follower arms are keyed to the outside of a load-bearing tube which in turn is keyed to the rods within the tube at a point or points along the bundle between the arrested end portions of the bundle. The arms bear against respective cams that are fixed to the load to be counterbalanced. Notably, the cams are parts that are to be mounted in the equipment to be controlled and the sockets keyed to the ends of the bundle of rods are locked in that equipment so as to resist the torsion that develops. The cams operate about pivots whose common axis is parallel to the bundle of rods. As the cams are forcibly turned, the cam-follower arms twist a portion of the bundle of rods between its ends about the axis of the bundle, developing torsion in the bundle of rods. The contour of the cams is related to the load so that the desired effort developed by the bundle of rods is applied to the load continuously along the cam surface. Energy is stored in the bundle rods, or delivered to the load, at rates determined by the cam contour.